Why did Jesus question God on the cross?

"Ahh" who do think David was quoting - especailly in psalm that ends with the messiah ruling the world - that psalm is speaking of a lot of things Who do you think David foreshadows in the journey up the mounth of Olives or his bringing the Ark into Jerusalem or in his flight to Jericho and so on King David is a very interesting character in the Bible - effectively standing half way between the two Adams (1 Cor 15 clearly teaching Jesus as the second Adam) - he is a very carnal man if you examine his life but he is also called a "man after God's own heart" there is a lot more going on with King David - juest like his psalm 22

I don't want to get off topic but we can't take the crucifixion by itself and ask one question as to why a thing happened or was said thinking only of that one event the entire Old testament is pointing to it and again and again it is being Prophesied and played out in peoples actions in bible history and in the rituals of the Jewish convenant

It will try and answer the question this way.

If the cross represents the place why God Judged, Adam, Eve and the serpent If it the place were God removed the fig leaves and gave Adam and Eve the coats of animal skins If if represents the place were God drove them out the Garden; What do you think they were screaming as the angels drove them out - do you think them might have felt "forsaken by God"?

If you read The Psalm 21 that Christ quotes here, you will see, that it is a prayer, that despite describing pain and abandonement is also full of hope. From the 19th verse David starts to petition God and surely believes that He will come and help him.

So if Christ prays with the words of Psalm 21 on the cross, He surely has in mind the future resurrection. And the first verse is here just a painful cry of what He feels in his human nature.

It is important to remember that the purpose of asking a question is often, but not always, to gain information, as is evidenced throughout the life of Jesus and even in our own lives.

Oftentimes as a child my mother would ask me if I had disobeyed her. She already knew the answer to that question, but there was something significant to me in having to answer that.

This specific cry from the Cross expresses the anguish of separation, but it also informs us that Jesus did, in fact, suffer separation from the Father. This is very important, because unless Jesus took upon Himself the entire penalty of our sin, then we would still remain something unpaid from our own penalty that would be left to us to pay. Indeed, apart from this cry of Jesus, we may have been left to wonder if He really experienced the full extent of the penalty for our sin.

So, we can't always assume that the purpose of every question is to gain information, implying a lack of knowledge within the one who asks. Jesus clearly understood the full extent of what He would endure, even though He expressed the depths of that pain.

I don't have a complete answer, but here is a partial one: two reasons I can think of:

To fulfill prophecy: Psalm 22:1

My God, my God, why have you forsaken me?

The rest of the Psalm seems to be largely about Jesus and especially His Death. The Psalm therefore shows us that this was not an accident but was God's plan.

In our place: if Jesus did not say it, or we do not appropriate His sacrifice in our lives, then I presume it would / will be us saying, "God, why have you abandoned me?" - presumably the cry of those in Hell.

It was a rhetorical question. Jesus was both fully man and fully God (Philippians 2:5-9) and, as evidenced by the verse you also mentioned (Matthew 26:39), He was conflicted about what was about to happen. He knew that He had to go through with it ("not mine but your will be done"), but His human nature did not want to die.

You also have to consider those three days that He would be buried before the Resurrection. The best Biblical reference to this is 1 Peter 3:18-20 which addresses that he was "made alive in the spirit". This suggests a death of the spirit. In the description of the Ascension by Luke (Acts 1:9-11), His spirit is taken up into Heaven, meaning He wasn't there when His body was killed.

I don't believe there is a consensus among scholars about where exactly Jesus' spirit went during that time, but it was not to Heaven. For the first time in Jesus' existence, He was to be separated from The Father. He and The Father and The Spirit are one and that separation must have been extremely painful to consider.

Jesus was the perfect man. But He was also man. He knew He would die, He knew it was to God's glory, and He knew that He would be apart from God for those three days. He was very conflicted. His last cry of anguish is the embodiment of that conflict and a clear representation of His humanity, but also His last hours are a clear representation of His Godliness. In the descriptions in Luke and John, His other "last words" are mentioned in which He asks The Father to forgive them, He tells the disciples to love His mother and His mother to love them, and He proclaims the Paradise awaiting them all with The Father.

So, to more directly answer your question: He did not have to ask The Father. He knew exactly why it had to be done. But He was still afraid.

I do not think that one reading the Gospel account could say that Jesus was wondering of the plan of God at this point in the salvation history. Here is how Jesus present himself in the good shepherd discourse :

14 I am the good shepherd. I know my own and my own know me, 15 just as the Father knows me and I know the Father; and I lay down my life for the sheep. 16 And I have other sheep that are not of this fold. I must bring them also, and they will listen to my voice. So there will be one flock, one shepherd. 17 For this reason the Father loves me, because I lay down my life that I may take it up again. 18 No one takes it from me, but I lay it down of my own accord. I have authority to lay it down, and I have authority to take it up again. This charge I have received from my Father.” The Holy Bible: English Standard Version. 2001 (Jn 10:14–18). Wheaton: Standard Bible Society. emphasis added

Jesus came to die on the cross. It was not part of the plan, but the plan all together. Here is how John the Baptist presents Jesus :

29 The next day he saw Jesus coming toward him, and said, “Behold, the Lamb of God, who takes away the sin of the world! The Holy Bible: English Standard Version. 2001 (Jn 1:29). Wheaton: Standard Bible Society.

The cross was the plan all along. Why did Jesus quote the psalm 22 on the cross? In doctrine there is a separation in the relationship between Jesus and God, when Jesus bored the sins on the cross. That was the first separation of Jesus with the Father since eternity past. That might be the reason why Jesus quotes this psalm.